1. Field of the Invention
The invention generally relates to networks in which power is delivered using Power over Ethernet (PoE) technology. More specifically, the invention is related to a system and method that allows a PoE-capable switch to selectively provide power to one or more remote devices in an instance where the power being supplied to the switch itself is limited or failing.
2. Background
Power over Ethernet (PoE) technology enables the transmission of electric power, along with data, to remote devices over standard twisted-pair cable in an Ethernet network. This technology is commonly used for powering Voice over Internet Protocol (VoIP) telephones, wireless local area network (WLAN) access points, webcams, embedded computers, and other appliances where it would be inconvenient or infeasible to supply power separately. The technology works with an unmodified Ethernet cabling infrastructure. The current industry standard for PoE technology is the IEEE 802.3af standard, although proprietary PoE systems exist as well.
A conventional PoE-capable switch is able to detect whether a device connected to one of its ports is capable of being powered via the Ethernet cable and to supply power to the device accordingly. However, such a switch is not capable of selectively providing power to one or more devices powered by the switch in an instance where the supply of power to the switch is limited or failing. This issue will be further explained with reference to FIG. 1 and FIG. 2.
FIG. 1 illustrates a conventional network system 100 in which a PoE-capable switch 102 is used to transfer data between a local area network 104 and a plurality of remote devices 106, as well as to provide power to those devices. As noted above, each of powered devices 106 may be one of a VoIP telephone, WLAN access point, webcam, embedded computer, or other appliance adapted to receive both data and power via a standard Ethernet cable. In network system 100, switch 102 is powered by AC power supplied by a local utility. When the AC power supply to switch 102 is shut off, as in the case of a power outage, switch 102 will shut down, thereby also depriving devices 106 of power. This may be extremely disruptive to the users of devices 106. For example, in an instance where a device 106 is a VoIP telephone, loss of power in this manner may lead to the unexpected termination of an active telephone conversation.
A conventional method for dealing with the foregoing issue is to place an uninterruptable power supply (UPS) between the utility power supply and the PoE-capable switch. Such an approach is depicted in network system 200 of FIG. 2. As shown in FIG. 2, a UPS 208 has been connected between the utility power supply and PoE-capable switch 102. In an instance where the utility power supply is shut off, UPS 208 can provide battery-supplied power to switch 102 until AC power from the utility is restored or until the UPS battery or batteries are depleted, whichever occurs first. Consequently, switch 102 can continue to provide power to devices 106 at least until the battery-supplied power runs out.
However, during the time that switch 102 is running off of UPS battery power, switch 102 is not aware that the utility power supply has failed and that the UPS batteries are now the primary source of power. As a result, switch 102 continues to provide power to all devices 106, whether such devices are critical or non-critical, as it normally would. By consuming power in this fashion, switch 102 and all devices 106 do not stay up very long. Furthermore, when power runs out, switch 102 and all devices 106 (whether critical or not) turn off simultaneously and without regard to priority. Where switch 102 requires a large power supply, the only way to avoid these issues is to install a very large UPS and optionally a generator between the utility power supply and switch 102, such that secondary power can be provided to switch 102 even during very long power outages. This is a costly proposition.
The issue of managing power delivered to a PoE switch that then delivers power to devices at the edge of network is becoming a critical one. This is due to rapid growth in the deployment of IP telephones, wireless access points, video cameras, and other devices that leverage PoE capabilities. This problem will be exacerbated in the future as more and more devices and systems are designed that rely on PoE technology. In accordance with the current 802.3af standard, 48 volts of DC power are provided over two out of four available pairs on a Category 3/Category 5 Ethernet cable with a maximum current of 400 milliamperes for a maximum load power of 15.4 watts. However, a future standard, commonly referred to as PoE+, is presently under development. This draft standard describes extending PoE technology by using all four pairs of standard Ethernet Category 5 cable to provide up to 56 watts of power. The higher power available with this future standard should make it possible to provide power to equipment with higher power requirements, such as WiMAX transmitters, pan-tilt-zoom cameras, IP-videophones and thin clients.
What is needed, then, is a system and method that would allow a PoE-enabled Ethernet switch to receive a notification from a UPS that a primary power source has failed such that the switch can selectively provide power to one or more of devices powered by the switch during the time that switch is running off of a secondary power source. For example, it would be beneficial if such a switch could selectively provide power to only those powered devices that are deemed critical and reduce or turn off power to those powered devices that are deemed non-critical in response to receiving such a notification from the UPS. This would allow the switch to provide power to the critical devices for a longer period of time in a manner that does not require investment in a larger, more expensive secondary power source.